The present invention relates to a developing method for electrophotography, and more particularly to a developing method wherein insulating toner is used and visible images which are excellent in the clearness of image, have no fog, and are rich in gradient can be obtained.
There has so far been known a jumping developing method wherein a one-component developer is used and the holder of such developer is positioned against the electrostatic image holder with a necessary clearance, and the development is made by the developer that flies over the clearance in an alternating electric field whose frequency is not more than 1 KHx (e.g. Japanese Patent Publication Open to Public Inspection No. 18656/1980). Further, there has been known the technology wherein a highfrequency pulse bias is impressed in the clearance between the electrostatic image holder and the developer holder confronting each other with such clearance, and thereby toners adhere to the image area of the electrostatic image holder but they do not adhere to the non-image area (e.g. U.S. Pat. Nos. 3,890,929, 3,866,574 and 3,893,418).
Such widely-known examples all employ one-component dry toner consisting of toner alone and utilize an arrangement wherein the electrostatic image holder and the toner holder are positioned to face each other with a clearance that requires a flying of toner such that an adherence of toner to the non-image area will be prevented.
Insulating toner, unlike conductive toner, has the great merit that it can be transferred to an untreated paper. One contact developing method has had a big problem from the occurrence of fog, though it has had merits such as (i) high speed developing is possible (ii) low voltage developing is possible and (iii) toner scattering is less compared with a non-contact developing method seen in a jumping development. The reason for this is that the insulating toner tends to adhere to the electrostatic image owing to its charge-induction property, residual voltage and triboelectrification because the insulating toner has an electric charge. As a fog-removing means, on the other hand, means using a magnetic force is generally employed. Strong magnetic bias can remove the fog but only from the edge portion of the image or from the low voltage portion; on the other hand, toner adheres from the scattering thereof and as a result the images lack the desired clearness and sharpness. If, on the other hand, a strong scraping force is used, the aforesaid clearness and sharpness are improved but the image quality in the developing direction will deteriorate due to the scattering of toner and the scraping force in the developing direction, which is a drawback.